Hi David Brenchley,
Indeed validation notifications provided by INotifyDataErrorInfo and (IDataErrorInfo + INotifyPropertyChanged) interfaces affects only visible items, because visual objects for items and cells are created only for visible ones.
You could use RowTemplate/StyleSelector which could be bound to a property which indicates that there is an error for this item, but again this will work only for visible items. You could by-pass this behavior if you turn off row and column virtualization. This is not recommended (but it is an option if you don't have large data with too much columns).
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